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| United States Patent |
5,040,470
|
|
Lofton
, et al.
|
August 20, 1991
|
Steam generating system with NO.sub.x reduction
Abstract
A method and system for flue gas recirculation is disclosed which will
minimize NOx production from hydrocarbon combustion. In the present
invention a furnace having an oxygen-bearing primary source of combustion
air, a mixing chamber, a combustion chamber in downstream communication
with the mixing chamber and an exhaust section downstream of the
combustion chamber is provided with a flue gas recirculation line. The
recirculation line establishes communication between the exhaust section
and the mixing chamber for the return of combustion products as a
secondary source of combustion air which is relatively lean in oxygen and
is combined with the primary source of combustion air in the mixing
chamber. The ratio of flow rates for the primary and secondary sources of
combustion air is controlled by a signal generated by a sensor which
senses the oxygen concentration in the mixing chamber.
| Inventors:
|
Lofton; Ruth E. (Bakersfield, CA);
Robinson; Dale E. (Bakersfield, CA);
Hutchinson; Daniel H. (Bakersfield, CA)
|
| Assignee:
|
Shell Western E&P Inc. (Houston, TX)
|
| Appl. No.:
|
614360 |
| Filed:
|
November 15, 1990 |
| Current U.S. Class: |
110/234; 110/188; 110/204; 122/135.1; 432/72; 432/222 |
| Intern'l Class: |
F23B 007/00 |
| Field of Search: |
432/222,72,180
110/188,204,234
122/135.1
|
References Cited
U.S. Patent Documents
| 3472498 | Oct., 1969 | Price et al. | 432/72.
|
| 3861334 | Jan., 1975 | Stockman | 110/204.
|
| 3947235 | Mar., 1976 | Bornert | 432/72.
|
| 4004875 | Jan., 1977 | Zink et al. | 431/9.
|
| 4162140 | Jul., 1979 | Reed | 239/431.
|
| 4182246 | Jan., 1980 | Lombana et al. | 110/188.
|
| 4253404 | Mar., 1981 | Leonard | 110/188.
|
| 4255132 | Mar., 1981 | Carthew | 432/72.
|
| 4297093 | Oct., 1981 | Morimoto et al. | 431/10.
|
| 4324545 | Apr., 1982 | Hubbert | 432/222.
|
| 4331086 | May., 1982 | Fitch et al. | 110/204.
|
| 4358268 | Nov., 1982 | Neville | 432/180.
|
| 4505665 | Mar., 1985 | Mansour | 431/10.
|
| 4568279 | Feb., 1986 | Logue et al. | 432/72.
|
| 4618323 | Oct., 1986 | Mansour | 431/177.
|
| 4715810 | Dec., 1987 | Ramsey et al. | 432/72.
|
| 4975047 | Dec., 1990 | Mitsuhashi et al. | 432/72.
|
Other References
Energy Technology Handbook, Douglas M. Considine, McGraw-Hill, 6/1977, pp.
9-335 to 9-348, "NOx Control by Furnace and Burner Design".
"Testing of Low-NOx Combustion Retrofit", L. L. Larsen and W. A. Carter,
KVB Report (KVB 71 60412-2067).
Transjet.RTM. Burner, Hague International (promotional literature).
"Introduction to COEN Low NOx Burner", COEN Company, Inc. (promotional
literature).
"COEN Low NOx Design Techniques Readily Solve Your Emission Problems",
(Technical Bulletin 20-102, COEN Co., Inc. (promotional literature).
"High-Performance, Lo-NOx.RTM. Energy-Miser Burners" (Bulletin 52LN),
National AirOil Burner Company, Inc.
"NOx Control for Gas-Fired Steam Generators", Energy and Environmental
Research Corporation (promotional literature).
|
Primary Examiner: Yuen; Henry C.
Attorney, Agent or Firm: Smith; Mark A.
Parent Case Text
This is a division of application Ser. No. 173,323, filed Mar. 25, 1988 now
U.S. Pat. No. 5,002,484.
Claims
What is claimed is:
1. A steam-generating device, comprising:
a furnace unit comprising:
an air intake regulating device which provides a primary source of
combustion air which is relatively rich in oxygen;
a mixing chamber in downstream communication with the air intake regulating
device;
a combustion chamber in downstream communication with the mixing chamber;
and
an exhaust section in downstream communication with the combustion chamber;
a flue gas recirculation system comprising:
a recirculation line establishing communication between the exhaust section
and the mixing chamber to provide a secondary source of combustion air to
the mixing chamber which is relatively lean in oxygen; and
a control system comprising:
a sensor for monitoring the oxygen concentration in the mixing chamber and
generating a signal which is a function thereof; and
a means responsive to the signal for controlling the flow rate of the flue
gas through the recirculation line; and
a heat exchange system comprising:
a heat exchanger in thermal communication with the furnace unit; and
a circulation system extending through the heat exchanger through which
water passes and is converted to steam.
2. A steam-generating device in accordance with claim 1 further comprising:
a second control system comprising:
a second sensor for monitoring the oxygen concentration in the flue gas and
generating a second signal which is a function thereof; and
a controlling unit for controlling the air intake regulating device
responsive to the second signal whereby the rate of flow of the primary
source of combustion air into the furnace unit is a function of the oxygen
concentration in the flue gas.
3. A hydrocarbon fired steam generator comprising:
a primary source of combustion air which contains a substantial oxygen
concentration;
a mixing chamber which receives the combustion air provided by the primary
source;
a combustion chamber in downstream communication with the mixing chamber;
a fuel inlet in communication with the combustion chamber;
an exhaust section in downstream communication with the combustion chamber;
a recirculation line establishing communication between the exhaust section
and the mixing chamber to provide a secondary source of combustion air for
communication with the primary source in the mixing chamber;
means for sensing the oxygen concentration in the combined combustion air
presented to the mixing chamber and generating a signal as a function
thereof;
means for controlling the flow rate of the secondary source of combustion
air which is responsive to the signal from the means for sensing the
oxygen concentration in the mixing chamber;
a second means for sensing oxygen concentration which is in sensory
communication with the exhaust gas and which generates a second control
signal which is a function thereof;
means for controlling the flow of the primary source of combustion air
which is responsive to the second control signal; and
a heat exchanger, comprising:
a water inlet;
a fluid circulation system in thermal communication with the combustion
reaction and in communication with the water inlet at one end; and
a steam outlet in communication with the other end of fluid circulation
system; whereby energy from the combustion reaction is transferred to the
water which is converted thereby from a liquid to a vapor phase.
4. A steam generator in accordance with claim 3 wherein the means for
controlling the flow rate of the secondary source of combustion air
comprises a valve in the recirculation line actuated by the signal from
the means for sensing the oxygen concentration in the mixing chamber.
Description
BACKGROUND OF THE INVENTION
The present invention relates to a system and method for reducing
pollutants from the combustion of hydrocarbon fuel and, more particularly,
to a system and method for recirculating flue gas in a controlled,
optimimized manner to minimize NOx formation as a product of hydrocarbon
combustion.
NOx is a common designation representing two oxides of nitrogen, nitric
oxide (NO) and nitrogen dioxide (NO.sub.2). Together, these compounds
react with hydrocarbons in the presence of oxygen and sunlight to form
photochemical smog. It is for this reason that environmental concerns and
attendant regulatory controls have required efforts to limit the amount of
NOx generated by the combustion of hydrocarbon fuels.
Hydrocarbon-fired steam generators used for enhanced oil recovery are
illustrative of this need and provide the preferred embodiment discussed
hereinafter. In such applications, multiple furnace units and attendant
steam generators are widely separated over an oil-bearing formation and
must use available hydrocarbon fuel to convert water to steam for
steam-flooding the underground formation. The feedstock fuel available is
most often unprocessed or minimally processed natural gas or crude oil.
Many different compounds may be present and mixed in such fuel, but a
typical natural gas mixture might include:
______________________________________
Component Volume %
______________________________________
CH.sub.4 92%
C.sub.2 H.sub.6 3%
C.sub.3 H.sub.8 1%
C.sub.4 H.sub.10 1%
Other Hydrocarbons
3%
______________________________________
Typical combustion in a furnace unit for an enhanced oil recovery steam
generator would yield combustion products as follows:
C.sub.A H.sub.B +O.sub.2 +N.sub.2 .fwdarw.CO.sub.2 +H.sub.2 O+N.sub.2
+NOx+CO
More specifically to point, the particular mechanism, thermal NOx
production, responsible for oxidizing nitrogen in the ambient combustion
air can be summarized as follows:
##STR1##
The elevated temperature within the furnace supplies the energy for oxygen
molecules to dissociate and, as the temperature rises into the range of
2,800.degree. to 3,000.degree. F., the oxygen free radicals have
sufficient energy to split bonds within the nitrogen molecules supplied by
the combustion air. One of these nitrogen atoms combines with the oxygen
and the other is sufficiently reactive to break another oxygen-oxygen
bond, thereby forming another NOx molecule and producing another oxygen
free radical to further propagate NOx production.
Without pollution controls, such combustion might yield NOx in the range of
0.06 to 0.1 pounds per million Btu fired.
However, it is known that recycling a portion of the combustion products in
the exhaust or flue gas dilutes the oxygen concentration presented in the
combustion air available for the combustion reaction and can significantly
reduce NOx production. A key mechanism in reducing the NOx concentration
is the effect that this dilution has on the temperature of the flame
within the furnace. Significantly increasing the amount of inert gas in
the combustion air increases the amount of gas which must be heated, but
does so without correspondingly increasing the amount of oxygen available
for combustion. Thus, the heat load drawing on the combustion reaction is
higher and the recycled flue gas serves to lower the temperature of the
flame within the furnace. This in turn reduces the formation of NOx as a
combustion product because the reactions necessary for NOx formation are
not favored by the lower reaction temperatures.
However, as discussed above, the NOx reduction is a sensitive function of
the temperature of the combustion reaction and is materially influenced
within a relatively narrow range. Thermal NOx production increases nearly
exponentially once the combustion temperature exceeds a critical
temperature in the range of 2,800.degree. to 3,000.degree. F. and
unmodified combustion materially exceeds this critical temperature while
ideal flue gas recirculation produces combustion temperature slightly
below this. Thus, too much oxygen and the reaction temperature, and
thereby the NOx concentration within the combustion products,
substantially increases. Conversely, insufficient oxygen produces
incomplete combustion which increases the concentration of carbon monoxide
and other undesirable pollutants and potentially destabilizes the
combustion reaction.
The prior art teaches control of the flue gas recirculation on a volumetric
basis, either directly metering the flow rate of the flue gas returned or
by performing a material balance utilizing the temperature of the flue
gas, ambient air, and blended combustion air along with a known capacity
for the blower drawing the ambient air into the furnace unit. A damper or
other manual or automatic control means in the recirculation lines is then
set based upon the calculated volume of recirculated flue gas. This may be
enhanced by directly metering the volume of flue gas returning through the
recirculation line to correspond to the calculated flow rate.
However, the prior art methods of reducing NOx produced are an indirect
approximation and are not responsive to the realities of dynamic
operation. Variations in the ambient temperature, furnace temperature,
fuel composition, load on the furnace, etc. all render the use of such
approximation techniques a crude tool to estimate the appropriate rate of
flue gas recirculation. Further, it is necessary that the setting be
substantially conservatively oxygen-rich in order to accommodate
variations and inaccuracies in estimates because running the furnace too
oxygen-lean risks unsafe and unstable combustion. Thus, the conservative
safety margins necessary to account for the variations discussed above
must be accommodated in a system and process that are very sensitive to
even small variations. This results in less than optimal performance and
materially increases the level of NOx produced during combustion.
The prior art has also approached reducing the NOx concentration in
combustion products by manually or automatically controlling the capacity
of the blower as a function of the concentration of unconsumed oxygen
appearing in the flue gas. While this does serve to decrease the absolute
amount of oxygen presented in the combustion air, it does nothing to alter
the thermal load by increasing the ratio of inert materials to oxygen in
the combustion air presented. Again, the commercially achievable results
have been limited.
SUMMARY OF THE INVENTION
It is therefore an object of the present invention to provide a system for
combustion of hydrocarbon fuel which monitors the amount of oxygen in the
combustion air for the purpose of maximizing the recirculation of low
oxygen flue gas into the combustion air, thereby lowering the temperature
of the combustion reaction and minimizing NOx production.
Another object of the present invention is to establish a controlled flue
gas recirculation which insures sufficient oxygen in the combustion air to
support stable combustion yet leans the oxygen concentration in order to
reduce NOx production.
Finally, it is an object of the present invention to improve steam
generators for enhanced oil recovery in which flue gas recirculation is
controlled to minimize NOx production yet ensure sufficient oxygen for
stable, efficient, and complete combustion of hydrocarbon fuel within the
furnace unit supplying the thermal energy for converting water into steam
for injection into a hydrocarbon reservoir.
Toward the fulfillment of these and other objects for establishing a
combustion system for hydrocarbon fuel, the present invention comprises a
furnace having an oxygen-bearing primary source of combustion air, a
mixing chamber, a combustion chamber in downstream communication with the
mixing chamber and an exhaust section downstream of the combustion
chamber. A recirculation line establishes communication between the
exhaust section and the mixing chamber for the return of combustion
products as a secondary source of combustion air which is relatively lean
in oxygen and is combined with the primary source of combustion air in the
mixing chamber. The ratio of flow rates for the primary and secondary
sources of combustion air is controlled by a signal generated by a sensor
which senses the oxygen concentration in the mixing chamber.
A BRIEF DESCRIPTION OF THE DRAWINGS
The above brief description as well as further objects, features and
advantages of the present invention will be more fully appreciated by
reference to the following detailed description of the
presently-preferred, but nonetheless illustrative, embodiment of the
present invention with reference to the accompanying drawings in which:
FIG. 1 is a schematic illustration of a steam generator incorporating the
present invention;
FIG. 2 is a block diagram of the control systems in a furnace unit
constructed in accordance with the present invention; and
FIGS. 3A and 3B are a flow diagram of the controlled flue gas recirculation
in a combustion process in accordance with the present invention.
A DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
FIG. 1 illustrates a steam generator 10 for use in enhanced oil recovery
which employs a hydrocarbon-driven furnace 12 to convert water to steam.
Furnace 12 is provided with a primary source of combustion air 14 in
communication with a blower 16 which feeds into a mixing chamber 18.
A combustion chamber 22 of furnace 12 is provided in downstream
communication with mixing chamber 18. Combustion chamber 22 will also
include fuel inlet 20 and an ignition device 24. An exhaust section 26 is
downstream of the combustion chamber and leads to a flue or stack 28.
A combustion system in accordance with the present invention includes a
flue gas recirculation system 32 which, in the preferred embodiment,
includes recirculation line 30 which provides communication between
exhaust section 26 and mixing section 18 of furnace 12. Further, flue gas
recirculation system 32 is provided with a means 40 for sensing the oxygen
concentration in the mixing chamber and generating a signal as a function
thereof. In the preferred embodiment oxygen is sensed directly with an
in-situ sampling type sensor, sensor 40A. Alternatively, a probe type
sensor can provide directly, or indirectly, a measure of the oxygen
concentration in the combined combustion air and other sensors will be
apparent to those skilled in the art upon consideration of the teachings
presented herein. A means 42 is provided for controlling the flow ratio of
the primary and secondary sources of combustion air which is provided to
mixing chamber 18 as a function of the signal from sensor 40A. In the
preferred embodiment, means 42 for controlling the flow ratio is provided
by a means for controlling the rate of flow for the recirculating flue gas
and includes a valve 44 actuated by a programmable logic controller 46A
which respond substantially independently of the flow rate of the primary
source of combustion air which is substantially stabilized at this stage
of operation. Other means for controlling the ratio between the primary
and secondary sources of combustion air will be apparent to those having
ordinary skill in the art, including substantially stabilizing the primary
source and otherwise adjusting a blower or other drive of the secondary
source, providing both sources of combustion air with separate blowers or
other drives and adjusting the relative speeds of the drives, adjusting
relative postions of valves, etc.
Flue gas recirculation system 32 thereby delivers a second source of
combustion air 34 through recirculation line 30 to mixing chamber 18. The
secondary source of combustion air 34 is characterized by a much lower
oxygen concentration and this oxygen-lean mixture of inert combustion
products is combined with the relatively oxygen-rich primary source of
combustion air 14 to produce blended or combined combustion air 36. In the
preferred embodiment, primary and secondary combustion air both enter the
suction line 18A of blower 16 which, together with the throat of the
furnace which leads to the combustion chamber, make up the mixing chamber.
However, a separate blower may be provided to the recirculating flue gas
or other modifications may be made to mixing chamber 18 which will be
apparent to those skilled in the art to provide a blending of the primary
and secondary sources of combustion air.
In the preferred embodiment, furnace 12 is provided with a second control
system 52 which includes a second means 54 for sensing the oxygen
concentration in the combustion products of flue gas 38. Second means 54
for sensing oxygen concentration, here sensor 54A, generates an output
signal which is a function of the oxygen concentration in the flue gas and
delivers this signal to a means 58 for controlling the flow of the primary
source of combustion air 14. A programmable logic controller 46B provides
the preferred control for valve 76 of control means 58.
Steam generator 10 is provided with a heat exchanger 60 which, in the
preferred embodiment, includes a radiant section 62 and a convection
section 64 through which water is heated as it passes from a source 66 to
an outlet 68. The water is preheated in convection section 64 then
converted to steam in radiant section 62 which exits outlet 68 and is
driven into a hydrocarbon bearing formation through an injection well.
FIG. 2 illustrates in greater detail the operation of the preferred
embodiment of flue gas recirculation system 32 and second control system
52 which reduce the NOx production of furnace 12 resulting from the
combustion of hydrocarbon fuel.
In operation, combustion air from primary source 14 is combined with
combustion air from secondary source 34 within mixing section or chamber
18 of the furnace. The primary source of combustion air is oxygen rich and
is most conveniently provided by the ambient air available at site, while
the secondary source of combustion air is oxygen-lean as provided by the
combustion products returned in the flue gas recirculation system. Thus,
the secondary source of combustion air serves to dilute the oxygen
concentration provided to the combined combustion air by the primary
source.
Fuel is combined with the combined combustion air and a combustion reaction
is initiated and sustained in combustion chamber 22 as illustrated by
flame 25 in FIG. 1, producing heat and combustion products. The heat is
used to perform useful work such as convert water to steam and the
combustion products are passed to an exhaust section from which a portion
of the exhaust or flue gas is expelled through a stack and the remaining
flue gas is drawn into flue gas recirculation system 32 at recirculation
line 30. See FIG. 2. In operation of the preferred embodiment of flue gas
recirculation system 32, the oxygen concentration of the combined
combustion air from the primary and secondary sources is sensed by sensor
40A which generates a signal which is passed to means 42 for controlling
the flow ratio of the primary and secondary sources of combustion air,
here provided by a programmable logic controller ("PLC") 46A which
compares the signal from sensor 40A with a predetermined set point
programmed into the PLC and schematically illustrated with reference
numeral 48 in this figure. Based on this comparison, an electronic signal
is passed to transducer 50 which converts the electronic signal to a
pneumatic actuating signal which directly actuates valve 44 within
recirculation line 30, thereby controlling the flow rate of the secondary
source of combustion air. Of course, application of the present invention
is not limited to pneumatically actuated valves. For instance, valve 44
may be hydraulically actuated and transducers 50 serve to convert the
signal to a hydraulic signal relayed to the valve, or solenoids may
directly throw valve 44 based upon an electronic signal. Alternatively,
the speed of a blower or other device provided may be adjusted as the
means 42 for controlling the flow ratio of the primary and secondary
sources of combustion air. Other variations will be apparent to those
skilled in the art familiar with the disclosure.
In the preferred embodiment, the set point 48 for PLC 46A is selected to
correspond to an oxygen concentration in the combined combustion air as
sensed by sensor 40A in the range of approximately 17-18 percent. As
discussed above, adjusting the flow rate of the secondary source of
combustion air with a substantially stabilized rate of flow from the
primary source is one means for adjusting the flow ratio of the combustion
air between the primary and secondary sources as a function of the signal
corresponding to the oxygen concentration of the combined combustion air.
Thus, in the preferred embodiment, controlling the rate of flue gas
recirculation provides fine tuning to minimize NOx production.
The primary source of combustion air is also regulated within the preferred
embodiment with secondary control system 52 in which the rate of flow for
the primary source of combustion air is controlled as a function of the
oxygen concentration sensed in the exhaust gas. Thus a second means for
sensing the oxygen concentration in the exhaust gas is provided by such
means as sensor 54A which generates a signal corresponding to the oxygen
concentration and sends that signal to a second PLC 46B within a means 58
for controlling the flow of the primary source of combustion air. Second
PLC 46B compares the signal from sensor 54A against a pre-programmed set
point schematically illustrated with reference numeral 72 in FIG. 2. The
second PLC 46B then generates an electronic signal which is a function of
this comparison and provides this signal to transducer 51 which converts
the signal to a pneumatic actuating signal which directly drives valve 76
to control the inflow of ambient air to the mixing chamber. As with
control means 42, many variations are within the scope of the invention
and means 58 for controlling the flow rate of the primary source of
combustion air is not limited to the presently preferred pneumatic valve
embodiment.
In the preferred embodiment, the set point 72 of PLC 46B corresponds to an
oxygen concentration of approximately 2 percent by volume remaining in the
combustion products of the exhaust or flue gas.
Various control and comparing functions have been set forth for
programmable logic controllers 46A and 46B. In the preferred embodiment,
each of these PLC's are provided by a single multi-function unit. Despite
the simplicity and convenience of this approach, it is noted that
alternatives will be apparent to those skilled in the art for generating
reference signals and comparing sensed signals with the reference signals
to generate appropriate control signals.
Various safety features are also provided in the steam generation of the
preferred embodiment which employ a third PLC 46C. This too can be
conveniently provided by the same PLC unit providing PLC's 46A and 46B.
See FIG. 1. PLC 46C senses the positions of limit structure in control
means 42 and 58 before initialing start up to ensure that flue gas
recirculation is closed and that the primary source of combustion air is
available. Start up will not initialize unless these conditions are
sensed. Further, a temperature sensor 85 monitors the temperature of the
operating furnace to shut the system down if the temperature of the
combined combustion air exceeds the rating of the blower.
FIGS. 3A and 3B illustrate a flow diagram of the preferred control scheme
for the present invention including certain safety features applicable to
the steam generator embodiment. This figure also provides the logic for
programming the multi-function PLC of the preferred embodiment. Before
start-up can be initiated, the limit switches must indicate that the
primary source of combustion air is available and that the secondary
source of combustion air through flue gas recirculation is shut down and
not available for initial combustion. If these conditions are sensed,
combustion can be initiated and an automatic delay system in the control
circuit allows the generator to reach full fire before the second control
system which monitors the exhaust gas is activated. The sensor is
activated and then compares the oxygen concentration in the stack gas with
a predetermined set point and will determine one of three conditions. If
the oxygen concentration in the stack gas is in the acceptable range
corresponding to the set point, the primary source of combustion air
remains at its current setting and maintains present availability. If
there is a variance between the set point and the oxygen concentration
sensed, the primary source of combustion air is adjusted. In either
instance, this monitoring activity repeats. In the third instance, this
comparison may demonstrate an excessively low oxygen concentration
indicative of substantial incomplete combustion. Upon sensing this
condition, the furnace unit will automatically shut down.
Once the oxygen concentration in the stack gas is substantially stabilized
in the range corresponding to the set point, a delay circuit is initiated
to insure stabilization. After this automatic delay, the sensor in sensory
communication with the combustion air is activated and transmits a signal
which is compared with a predetermined set point. If the oxygen
concentration sensed is within the range selected for the set point, the
flow rate for the secondary source of combustion air is maintained at the
current rate. However, if there is a variance, then the flow rate of the
secondary source of combustion air is adjusted accordingly. In each
instance this monitoring process continues. Further, an additional safety
feature provides for checking the temperature in the combustion air and
shutting down the furnace unit if it is too hot. Similarly, if this
temperature is satisfactory, then operation of the furnace will be
maintained and the monitoring will continue to insure operation within an
acceptable temperature range.
It is estimated that the present invention will reduce NOx yield to the
range of 0.03 to 0.05 pounds per million Btu fired. This is a substantial
reduction available by active control to continuously minimize NOx
production based on real time conditions rather than the selection of
conservative average conditions.
In the presently preferred embodiment of the method and system for flue gas
recirculation, as embodied in the illustrated steam generator, the
following components have been deployed by the applicants:
______________________________________
TABLE OF COMPONENTS
MANUFACTURE AND
ELEMENT MODEL
______________________________________
Programmable Logic Controller
Westinghouse PC-1100
(PLC) 46A, 46B and 46C
Second Sensor 54A Thermox WDG - III
(O.sub.2 in Stack Gas)
First Sensor 40A Thermox FCA
(O.sub.2 in Blended Combustion Air)
Valve 44 North American #1147-10
(means for controlling
(valve)
secondary source of
North American #1600-5-AP
combustion air 42)
(actuator)
Valve 76 North American #1156-16
(means for controlling
(valve)
primary source of North American #1600-5-AP
combustion air 58)
(actuator)
Transducers 50, 51
Brandt # PICPT2131
______________________________________
The foregoing components are merely illustrative of one embodiment of the
present invention and many variations of the present invention are
expressly set forth in the preceding discussion. Further, other
modifications, changes, and substitutions are intended in the foregoing
disclosure, and in some instances some features of the invention will be
employed without a corresponding use of other features. Accordingly, it is
appropriate that the appended claims be construed broadly and in a manner
consistent with the spirit and scope of the invention herein.
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